This invention relates to a modular conveyor chain, and more particularly to an improved chain link for use in constructing a modular conveyor chain.
Manufacturing and production facilities utilize modular conveyor chains to transport products or articles of production from one location to another. Conventional modular conveyor chains are typically comprised of multiple widths of thermoplastic chain links or modules. The links making up the modular conveyor chain typically each have a plurality of spaced link ends which intermesh with complementary spaced link ends projecting from an adjacent link or links. The individual chain links are typically similar in width and may be arranged in a bricked configuration. The intermeshing link ends are joined or hinged together by a connecting pin that permits the chain links to pivot with respect to each other. The chain links are joined together to form an endless conveyor chain that is typically driven by a drive sprocket.
In use modular chains are commonly subjected to tensile forces that tend to separate the individual chain links when the chain is placed under a tensile load. Tensile forces on the modular chain are transmitted from the chain link ends to the connecting pin and from the connecting pin to the link ends of the next link.
Conventional modular chain links are typically made of thermoplastic (e.g., acetyl, polyester, nylon and polypropylene). Chain links molded from plastic are much lighter than conventional metal links. The plastic chain links also include lower friction surfaces that engage the drive sprocket and the wear strips supporting the conveyor during operation of the conveyor. These lower friction surfaces reduce the amount of friction force that is exerted on the chain links. Lowering the friction force exerted on the conveyor reduces the tension on the chain links and minimizes chain link stretching.
The choice of the thermoplastic used for the chain link usually depends on the mechanical properties which are desired (i.e., tensile strength, low-friction, chemical resistance and/or suitability for use under extreme cyclic temperatures). The strength of the modular chain link is especially important because chain links with increased mechanical properties increase the tensile strength of the modular conveyor chain and reduces the stretching that may result due to loading.
Modular conveyor chains are often used to carry goods from one location to another location where the temperature of the environment at the two locations is significantly different. The individual chain links expand as the temperature of the chain increases, and contract as the temperature of the chain decreases. As the individual chain links expand or contract the overall length of the conveyor chain varies significantly as a result of a high co-efficient of thermal expansion that is typically associated with thermoplastics.
Chain expansion due to transporting goods into a high temperature environment can be especially problematic because the interaction between the chain and the drive sprocket can be compromised. As the operating temperature of the individual chain links increases, the overall tensile strength of the conveyor chain may be reduced because thermoplastic chain links typically lose tensile strength at elevated temperatures. The Modulus of Elasticity also decreases for most plastics at elevated temperatures such that plastic chain links stretch when they are placed under tension in a high temperature operating environment.
There are also problems associated with using modular conveyor chains to transport items into a low temperature environment (e.g., in a freezer). The individual chain links contract as they enter the low temperature environment. This contraction negatively effects the interaction between the chain and drive sprocket. The conveyor chain tends to bind on the sprocket leading to wear and compromising the useful life of the modular conveyor chain.
Modular conveyor chains are also used in applications where endless conveyor chains are stacked one on top of another. One typical example is in a pasteurizer in a brewery. The modular conveyor chains are used to transport large numbers of bottles or cans through a pasteurizer. The length of the modular conveyor chains in these multi-deck systems increases significantly as the chain travels through the high temperature environment in the pasteurizer. As the chain length increases, the bottom catenary section of an upper conveyor sags and may interfere with products positioned on a lower conveyor.
Additional metal links are often used in conjunction with the thermoplastic chain links. In this type of configuration a pattern of metal links is formed throughout the modular conveyor chain. This combination of thermoplastic links and metal links results in loads on the conveyor being carried primarily by the metal links. One of the problems associated with combining links made from two different materials in order to form a modular conveyor chain is that there are significant bending stresses generated within the thermoplastic chain links due to the differences in the coefficient of friction and coefficient of thermal expansion between the thermoplastic chain links and the metal chain links.
Some conventional conveyor chains that include thermoplastic chain links are disclosed in U.S. Pat. Nos. 5,586,644, 5,575,937, 5,137,144, 4,893,709, 4,711,346, 4,698,504, 4,490,970, and 4,383,818.